Inoculum effect of methicillin-susceptible Staphylococcus aureus against broad-spectrum beta-lactam antibiotics

Staphylococcus argenteus bacteremia in the Republic of Korea

In 2015, Staphylococcus argenteus and Staphylococcus schweitzeri were proposed as new species, distinct from Staphylococcus aureus and collectively referred to as the S. aureus complex. However, no clinical reports of these new species exist in Korea. Upon the application of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for all bloodstream isolates since September 2022, S. argenteus was identified in one patient. Therefore, we aimed to search for new species among the archives of the S. aureus bacteremia cohort and describe their clinical and microbiological characteristics. Among the 691 archived S. aureus isolates between 2012 and 2018, one was identified as S. argenteus via MALDI-TOF MS. Both S. argenteus isolates (one in 2022) were obtained from patients with extensive pneumonia accompanied by bacteremia and both cases had fatal outcomes. They harbored multiple virulence genes (clfA, clfB, fnbpA, sdrC, sdrD, sdrE, bbp, cna, see, seg, sei, blaZ, fnbpB, and map) but did not harbor mecA and pvl. No matched sequence type (ST) was found in either isolate, and both S. argenteus isolates were closely related to ST1594, ST1593, ST1793, and ST1303, which belonged to S. argenteus. S. argenteus accounted for <1% of the S. aureus complex but had clinical characteristics similar to S. aureus. Therefore, clinicians should be aware of these factors to avoid misidentifying these strains as coagulase-negative staphylococci, and appropriate reporting is required to minimize confusion.IMPORTANCEStaphylococcus argenteus, a member of Staphylococcus aureus complex, has been reported as an important pathogen that causes clinically invasive infections in humans similar to S. aureus. Clinical isolates of S. argenteus have been reported across the world, showing a large geographical difference in prevalence and genomic profile. However, there have been no clinical reports regarding this new species in Korea. This is the first report to investigate the clinical and genetic characteristics of S. argenteus identified in patients with bacteremia, and the proportion of S. argenteus bacteremia among S. aureus bacteremia cohort in Korea.

Epidemiology and impact of methicillin-sensitive Staphylococcus aureus with β-lactam antibiotic inoculum effects in adults with cystic fibrosis

Staphylococcus aureus is the most prevalent cystic fibrosis (CF) pathogen. Several phenotypes are associated with worsened CF clinical outcomes including methicillin-resistance and small-colony-variants. The inoculum effect (IE) is characterized by reduced β-lactam susceptibility when assessed at high inoculum. The IE associates with worse outcomes in bacteremia and other high-density infections, and may therefore be relevant to CF. The prevalence of IE amongst a CF cohort (age ≥18 years), followed from 2013 to 2016, was investigated. Yearly methicillin-sensitive S. aureus (MSSA) isolates were screened at standard (5 × 105 CFU/mL) and high (5 × 107 CFU/mL) inoculum against narrow-spectrum anti-Staphylococcal β-lactams and those with anti-pseudomonal activity common to CF. A ≥ 4-fold increase in minimum inhibitory concentration between standard and high inoculum defined IE. Isolates underwent blaZ sequencing and genotyping and were compared against published genomes. Fifty-six percent (99/177) of individuals had MSSA infection. MSSA was observed at ≥105 CFU/mL in 44.8% of entry sputum samples. The prevalence of the IE was 25.0%-cefazolin; 13.5%-cloxacillin; 0%-meropenem; 1.0%-cefepime; 5.2%-ceftazidime; and 34.4%-piperacillin-tazobactam amongst baseline MSSA isolates assessed. blaZ A associated with cefazolin IE (P = 0.0011), whereas blaZ C associated with piperacillin-tazobactam IE (P < 0.0001). Baseline demographics did not reveal specific risk factors for IE-associated infections, nor were long-term outcomes different. Herein, we observed the IE in CF-derived MSSA disproportionally for cefazolin and piperacillin-tazobactam and this phenotype strongly associated with underlying blaZ genotype. The confirmation of CF being a high density infection, and the identification of high prevalence of MSSA with IE in CF supports the need for prospective pulmonary exacerbation treatment studies to understand the impact of this phenotype.

Comparable Effectiveness of Cefuroxime and Piperacillin-Tazobactam as Empirical Therapy for Methicillin-Susceptible Staphylococcus aureus Bacteremia

Our objective was to examine whether empirical antimicrobial therapy (EAT) against methicillin-susceptible Staphylococcus aureus bacteremia (MS-SAB) with piperacillin-tazobactam (TZP), cefuroxime or combination therapy with one of these was differentially associated with 7-, 30-, and 90- day all-cause mortality or MS-SAB relapse. A multicenter retrospective cohort study of adults with MS-SAB from 2009 through 2018 was used, and 7-, 30-, 90-day mortality and relapse within 90 days were assessed and expressed as hazard ratio (HR) with a 95% confidence interval (95% CI) using Cox proportional hazard regression analysis. Matching of the two monotherapy groups was performed using propensity score matching. In total, 1158 MS-SAB cases were included and received one of three EAT regimens: TZP (n = 429), cefuroxime (n = 337), or TZP or cefuroxime with one or more additional effective antimicrobial (n = 392). The overall 30-day mortality was 28.0% (25.5 to 30.3%). After adjustment and matching, there was no significant difference in 7-, 30-, or 90-day mortality between the therapy groups. The matched HR of death was 0.81 (95% CI, 0.38 to 1.76) at 7 days, 0.82 (95% CI, 0.47 to 1.46) at 30 days, and 0.81 (95% CI, 0.50 to 1.32) at 90 days for TZP compared with cefuroxime. Adjusted HR of 90-day relapse was insignificant between the three therapy groups: TZP: 1.55 (95% CI, 0.54 to 4.43); combination therapy: 1.73 (95% CI, 0.62 to 4.80) compared to cefuroxime. There was no significant difference in 7-, 30-, or 90-day mortality or relapse between MS-SAB patients treated with empirical TZP or cefuroxime after adjustment and matching of covariables.

IMPORTANCE This multicenter retrospective matched cohort study evaluated the effect of empirical antimicrobial therapy on the clinical outcome of methicillin-susceptible Staphylococcus aureus bacteremia (MS-SAB) in >1100 adult patients. To the best of our knowledge, this is the largest study to date evaluating the effect of empirical treatment on the MS-SAB outcome. Importantly, the study found no significant difference in either short- or long-term mortality nor relapse between patients with MS-SAB receiving empirical treatment with cefuroxime or piperacillin-tazobactam. As such, this study provides crucial contemporary data supporting the widespread clinical practice of initiating empirical antimicrobial therapy of sepsis with β-lactam-β-lactamase-inhibitor.

Staphylococcus aureus in Non-Cystic Fibrosis Bronchiectasis: Prevalence and Genomic Basis of High Inoculum Beta-Lactam Resistance

Rationale The pathobiology of Staphylococcus aureus in non-cystic fibrosis bronchiectasis (nCFB) is poorly defined. When present at high density or "inoculum", some methicillin-sensitive S. aureus (MSSA) can inefficiently degrade anti-Staphylococcal beta-lactam antibiotics via BlaZ penicillinases (termed, the inoculum effect). Given the high burden of organisms in bronchiectatic airways, this is particularly relevant. Objectives Drawing from a prospectively-collected biobank, we sought to understand the prevalence, natural history, potential for transmission, and antibiotic resistance profiles amongst nCFB-derived MSSA isolates. Methods All individuals attending a regional consultancy nCFB clinic with sputum collected between 1981-2017 were considered, and those with ≥1 S. aureus-positive culture comprised the cohort. Each individual's most recent biobank isolate was subjected to whole genome sequencing (including the blaZ gene), antibacterial susceptibility testing, and comparative beta-lactam testing at standard (5 x 105CFU/mL) and high (5 x 107CFU/mL) inoculum to assess for the inoculum, and pronounced inoculum effect (IE and pIE, respectively). Results Seventy-four of 209 (35.4%) individuals had ≥1 sputum sample(s) with S. aureus (68 MSSA, 6 MRSA). Those with S. aureus infection were more likely to be female. Amongst 60/74 MSSA isolates subjected to WGS, no evidence of transmission was identified, although specific MLST types were prevalent including ST-1, ST-15, ST-30, and ST-45. Antibiotic resistance was uncommon except for macrolides (~20%). Amongst the 60 MSSA, prevalence of IE and pIE, respectively, were observed to be drug specific; meropenem (0%, 0%), cefepime (3%, 5%), ceftazidime (8%, 0%), cloxacillin (12%, 0%), cefazolin (23%, 0%) and piperacillin-tazobactam (37%, 17%). The cefazolin IE associated with blaZ type A (p<0.01) and ST-30 (p<0.01), whereas the piperacillin-tazobactam IE associated with type C blaZ (p<0.001) and ST-15 (p<0.05). Conclusions S. aureus infection was common, although no evidence of transmission was apparent in our nCFB cohort. While routine susceptibility testing did not identify significant resistance, inoculum-related resistance was found to be relevant for commonly used nCFB antibiotics including cefazolin and piperacillin-tazobactam. Given previous associations between IEs and negative patient outcomes, further work is warranted to understand how this phenotype impacts nCFB disease progression.

β-Lactam Resistance in Upper Respiratory Tract Pathogens Isolated from a Tertiary Hospital in Malaysia

The rise of antimicrobial resistance (AMR) among clinically important bacteria, including respiratory pathogens, is a growing concern for public health worldwide. Common causative bacteria for upper respiratory tract infections (URTIs) include Streptococcus&nbsp;pneumoniae and Haemophilus influenzae, and sometimes Staphylococcus aureus. We assessed the β-lactam resistant trends and mechanisms of 150 URTI strains isolated in a tertiary care hospital in Kuala Lumpur Malaysia. High rates of non-susceptibility to penicillin G (38%), amoxicillin-clavulanate (48%), imipenem (60%), and meropenem (56%) were observed in S. pneumoniae. Frequent mutations at STMK and SRNVP motifs in PBP1a (41%), SSNT motif in PBP2b (32%), and STMK and LKSG motifs in PBP2x (41%) were observed in S. pneumoniae. H. influenzae remained highly susceptible to most β-lactams, except for ampicillin. Approximately half of the ampicillin non-susceptible H. influenzae harboured PBP3 mutations (56%) and only blaTEM was detected in the ampicillin-resistant strains (47%). Methicillin-susceptible S. aureus (MSSA) strains were mostly resistant to penicillin G (92%), with at least two-fold higher median minimum inhibitory concentrations (MIC) for all penicillin antibiotics (except ticarcillin) compared to S. pneumoniae and H. influenzae. Almost all URTI strains (88-100%) were susceptible to cefcapene and flomoxef. Overall, β-lactam antibiotics except penicillins remained largely effective against URTI pathogens in this region.

Ceftriaxone versus antistaphylococcal antibiotics for definitive treatment of methicillin-susceptible Staphylococcus aureus infections: a systematic review and meta-analysis

Optimal therapy for methicillin-susceptible Staphylococcus aureus (MSSA) infections is unclear. Current standard of care consists of antistaphylococcal antibiotics (ASAs) such as nafcillin, oxacillin and cefazolin. Ceftriaxone has been evaluated due to its advantage as a once-daily outpatient regimen. However, questions remain regarding its efficacy compared with ASAs. We aimed to conduct a review and synthesis of available literature for outcomes of patients treated with ceftriaxone or ASAs for MSSA infections. We searched Cochrane Central Register of Controlled Trials, Embase Ovid, MEDLINE Ovid, Scopus and Web of Science (1990 to June 2021). Risk of bias for cohort studies was assessed by the Newcastle-Ottawa scale. We pooled risk ratios (RRs) using the DerSimonian-Laird random-effects model for outcomes of those receiving ceftriaxone versus ASAs. Heterogeneity was assessed by the I² index. From 459 identified studies, 7 were included in the quantitative synthesis totalling 1640 patients. Definitive therapy with ceftriaxone was associated with a lower risk of toxicity requiring therapy alteration (RR 0.49, 95% CI 0.27-0.88; I² = 0%). There was no difference in terms of 90-day all-cause mortality (RR 0.93, 95% CI 0.46-1.88; I² = 9%), hospital readmission (RR 0.96, 95% CI 0.57-1.64; I² = 0%) or infection recurrence (RR 1.04, 95% CI 0.63-1.72; I² =0%). Current evidence suggests there is no difference in efficacy between ceftriaxone and ASAs for MSSA infection, with a lower risk of toxicity with ceftriaxone. Within the limitations of available retrospective studies, ceftriaxone is a consideration for definitive therapy of MSSA infection. [Trial registration: PROSPERO ID: CRD42021259086].

Novel human in vitro vegetation simulation model for infective endocarditis

Infective endocarditis (IE) is a heart valve infection with high mortality rates. IE results from epithelial lesions, inducing sterile healing vegetations consisting of platelets, leucocytes, and fibrin that are susceptible for colonization by temporary bacteremia. Clinical testing of new treatments for IE is difficult and fast models sparse. The present study aimed at establishing an in vitro vegetation simulation IE model for fast screening of novel treatment strategies. A healing promoting platelet and leucocyte-rich fibrin patch was used to establish an IE organoid-like model by colonization with IE-associated bacterial isolates Staphylococcus aureus, Streptococcus spp (S. mitis group), and Enterococcus faecalis. The patch was subsequently exposed to tobramycin, ciprofloxacin, or penicillin. Bacterial colonization was evaluated by microscopy and quantitative bacteriology. We achieved stable bacterial colonization on the patch, comparable to clinical IE vegetations. Microscopy revealed uneven, biofilm-like colonization of the patch. The surface-associated bacteria displayed increased tolerance to antibiotics compared to planktonic bacteria. The present study succeeded in establishing an IE simulation model with the relevant pathogens S. aureus, S. mitis group, and E. faecalis. The findings indicate that the IE model mirrors the natural IE process and has the potential for fast screening of treatment candidates.

Magnetic thermosensitive polymer composite carrier with target spacing for enhancing immobilized enzyme performance

A novel magnetic thermosensitive polymer composite carrier with target spacing was developed. In this strategy, thermosensitive polymer grafted on magnetic Fe₃O₄ for enhancing immobilized penicillin G acylase (PGA) performance and introduce immobilized target spacing into magnetic carriers for the first time. Fe₃O₄ nanoparticles were synthesized by a reverse microemulsion method. The modifier used was the silane coupling agent γ-methylacryloxypropyl trimethoxysilane (KH570) and then reacting with a reversible-adaptive fragmentation chain transfer (RAFT) reagent, 2-cyano-2-propyldodecyl trithiocarbonate (CPDTC). The thermo-sensitive nanoparticle-composite carrier of Fe₃O₄-grafted-poly N, N-diethyl acrylamide-block-poly β-Hydroxyethyl methacrylate-block-random copolymer of glycidyl methacrylate and methyl methacrylate (Fe₃O₄-g-PDEA-b-PHEMA-b-P(MMA-co-GMA)) were synthesized by RAFT polymerization technique that used N, N-diethyl acrylamide (DEA), β-Hydroxyethyl methacrylate (HEMA), Glycidyl methacrylate (GMA) and Methyl methacrylate (MMA) as monomer, then which were employed as functional carriers for the immobilization of PGA. Within the carrier, the epoxy group of GMA segment was a target immobilization site for PGA and the introduction of MMA reflected the target space of immobilized PGA to improve catalytic activity and catalytic activity recovery rate of the immobilized PGA. Characterizations demonstrated that the triblock copolymers grafted Fe₃O₄ nanoparticles were successfully fabricated by the structure design. Besides, under these circumstances the enzyme activity (EA), enzyme loading capacity (ELC) and catalytic activity recovery ration (CAR) reached 31235 U/g, 128.39 mg/g and 93.32 %, respectively. The catalytic activity of immobilized PGA maintained 87.4 % of initial value and the recovery ratio (R) of immobilized PGA reached 96.22 % after recycling 12 times. Furthermore, the immobilized PGA exhibited advantages of low temperature homogeneous catalysis and magnetic separation, which indicated broad application prospects in the biocatalysts' field.

Anti-biofilm Approach in Infective Endocarditis Exposes New Treatment Strategies for Improved Outcome

Infective endocarditis (IE) is a life-threatening infective disease with increasing incidence worldwide. From early on, in the antibiotic era, it was recognized that high-dose and long-term antibiotic therapy was correlated to improved outcome. In addition, for several of the common microbial IE etiologies, the use of combination antibiotic therapy further improves outcome. IE vegetations on affected heart valves from patients and experimental animal models resemble biofilm infections. Besides the recalcitrant nature of IE, the microorganisms often present in an aggregated form, and gradients of bacterial activity in the vegetations can be observed. Even after appropriate antibiotic therapy, such microbial formations can often be identified in surgically removed, infected heart valves. Therefore, persistent or recurrent cases of IE, after apparent initial infection control, can be related to biofilm formation in the heart valve vegetations. On this background, the present review will describe potentially novel non-antibiotic, antimicrobial approaches in IE, with special focus on anti-thrombotic strategies and hyperbaric oxygen therapy targeting the biofilm formation of the infected heart valves caused by Staphylococcus aureus. The format is translational from preclinical models to actual clinical treatment strategies.

Microscopic Analysis of Bacterial Inoculum Effect Using Micropatterned Biochip

Antimicrobial resistance has become a major problem in public health and clinical environments. Against this background, antibiotic susceptibility testing (AST) has become necessary to cure diseases in an appropriate and timely manner as it indicates the necessary concentration of antibiotics. Recently, microfluidic based rapid AST methods using microscopic analysis have been shown to reduce the time needed for the determination of the proper antibiotics. However, owing to the inoculum effect, the accurate measurement of the minimal inhibitory concentration (MIC) is difficult. We tested four standard bacteria: Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Enterococcus faecalis, against five different antibiotics: piperacillin, cefotaxime, amikacin, levofloxacin, and ampicillin. The results showed that overall, the microfluidic system has a similar inoculum effect compared to the conventional AST method. However, due to the different testing conditions and determination protocols of the growth of the microfluidic based rapid AST, a few results are not identical to the conventional methods using optical density. This result suggests that microfluidic based rapid AST methods require further research on the inoculum effect for practical use in hospitals and can then be used for effective antibiotic prescriptions.

Cefazolin and Ertapenem Salvage Therapy Rapidly Clears Persistent Methicillin-Susceptible Staphylococcus aureus Bacteremia

Cefazolin and ertapenem combination therapy was used successfully to salvage 11 cases (6 endocarditis) of persistent methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia, including immediate clearance (≤24 hours) in 8 cases. While in vitro synergy was modest, cefazolin plus ertapenem exhibited synergistic action in a rat model of MSSA endocarditis. The combination of cefazolin and ertapenem provides potent in vivo activity against MSSA beyond what is predicted in vitro and warrants further clinical study in the treatment of refractory MSSA bacteremia and endocarditis.

Inoculum effect of β-lactam antibiotics

The phenomenon of attenuated antibacterial activity at inocula above those utilized for susceptibility testing is referred to as the inoculum effect. Although the inoculum effect has been reported for several decades, it is currently debatable whether the inoculum effect is clinically significant. The aim of the present review was to consolidate currently available evidence to summarize which β-lactam drug classes demonstrate an inoculum effect against specific bacterial pathogens. Review of the literature showed that the majority of studies that evaluated the inoculum effect of β-lactams were in vitro investigations of Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Haemophilus influenzae and Staphylococcus aureus. Across all five pathogens, cephalosporins consistently displayed observable inoculum effects in vitro, whereas carbapenems were less susceptible to an inoculum effect. A handful of animal studies were available that validated that the in vitro inoculum effect translates into attenuated pharmacodynamics of β-lactams in vivo. Only a few clinical investigations were available and suggested that an in vitro inoculum effect of cefazolin against MSSA may correspond to an increased likeliness of adverse clinical outcomes in patients receiving cefazolin for bacteraemia. The presence of β-lactamase enzymes was the primary mechanism responsible for an inoculum effect, but the observation of an inoculum effect in multiple pathogens lacking β-lactamase enzymes indicates that there are likely multiple mechanisms that may result in an inoculum effect. Further clinical studies are needed to better define whether interventions made in the clinic in response to organisms displaying an in vitro inoculum effect will optimize clinical outcomes.

The increased frequency of methicillin-resistant Staphylococcus aureus with low MIC of beta-lactam antibiotics isolated from hospitalized patients

Methicillin-resistant Staphylococcus aureus (MRSA) causes severe infectious diseases and can be life-threatening in healthcare-settings. MRSA is classified into health-care associated (HA)-MRSA strains and community acquired (CA)-MRSA strains based on genotype and phenotype. CA-MRSA has been reported to show the lower minimal inhibitory concentration (MIC) of some antibiotics as compared to HA-MRSA. Recently, the prevalence of CA-MRSA has been increased in worldwide. CA-MRSA is isolated not only from the healthy individuals in a community but also from the patients in healthcare settings. However, the changing trend in frequency of HA-MRSA and CA-MRSA in the hospital setting is not clear. Therefore, we analyzed the trend of MIC to speculate the frequency of HA-MRSA and CA-MRSA in the facility. Moreover, gene mutations were evaluated on resistant gene loci with next generation sequencer. The frequency of strains with low MIC of beta-lactam antibiotics was gradually increased in isolated MRSA strains from the hospitalized patients. Whole genome analysis revealed the frequency of gene mutation was also decreased in some resistant loci, such as blaZ and blaR1. These findings highlight the changing trend of MRSA strains isolated from hospitalized patients.

Comparative Effectiveness of Exclusive Exposure to Nafcillin or Oxacillin, Cefazolin, Piperacillin/Tazobactam, and Fluoroquinolones Among a National Cohort of Veterans With Methicillin-Susceptible Staphylococcus aureus Bloodstream Infection

OBJECTIVE

Beta-lactam antibiotics are recommended as first-line for treatment of methicillin-sensitive Staphylococcus aureus (MSSA) bacteremia. The objective of this study was to compare effectiveness of anti-MSSA therapies among bacteremia patients exclusively exposed to 1 antimicrobial.

METHOD

This was a national retrospective cohort study of patients hospitalized in Veterans Affairs medical centers with MSSA bacteremia from January 1, 2002, to October 1, 2015. Patients were included if they were treated exclusively with nafcillin, oxacillin, cefazolin, piperacillin/tazobactam, or fluoroquinolones (moxifloxacin and levofloxacin). We assessed 30-day mortality, time to discharge, inpatient mortality, 30-day readmission, and 30-day S. aureus reinfection. Hazard ratios (HRs) and 95% confidence intervals (CI) were calculated using propensity-score (PS) matched Cox proportional hazards regression model.

RESULTS

When comparing nafcillin/oxacillin (n = 105) with cefazolin (n = 107), 30-day mortality was similar between groups (PS matched n = 44; HR, 0.67; 95% CI, 0.11-4.00), as were rates of the other outcomes assessed. As clinical outcomes did not vary between nafcillin/oxacillin and cefazolin, they were combined for comparison with piperacillin/tazobactam (n = 113) and fluoroquinolones (n = 103). Mortality in the 30 days after culture was significantly lower in the nafcillin/oxacillin/cefazolin group compared with piperacillin/tazobactam (PS matched n = 48; HR, 0.10; 95% CI, 0.01-0.78), and similar when compared with fluoroquinolones (PS matched n = 32; HR, 1.33; 95% CI, 0.30-5.96).

CONCLUSIONS

In hospitalized patients with MSSA bacteremia, no difference in mortality was observed between nafcillin/oxacillin and cefazolin or fluoroquinolones. However, higher mortality was observed with piperacillin/tazobactam as compared with nafcillin/oxacillin/cefazolin, suggesting it may not be as effective as a monotherapy in MSSA bacteremia.